Mobile terminals, such as wireless telephones, generally require that a specified radio frequency (RF) output power be delivered to a radiating antenna. Further, many such systems are required to control the transmitted power to achieve a specific level depending upon signal strength. To meet these requirements, system architectures generally incorporate a closed-loop power control scheme. Typically, this scheme requires sampling the output of a power amplifier to create a signal, which is sent to control circuitry. The control circuitry generates a control signal that adjusts the output power of the amplifier until it is within the specified power level. Such sampling of the output power is disadvantageous in that it increases the insertion loss between the output of the power amplifier and the radiating antenna. Therefore, sampling of the output power increases the required output power from the power amplifier and reduces battery life, which in turn reduces talk and standby time of the mobile terminal.
A common technique for sampling the output power includes the use of a directional coupler on the output of the power amplifier. The power coupled from the main signal path is diode detected to generate a signal proportional to the output voltage delivered to the antenna. Use of directional couplers, however, adds loss to the system, forcing the power amplifier to deliver more power, thereby reducing the battery life. In typical applications, the aforesaid loss is often 5–10% of the power amplifier output power and relates to significant loss in talk and standby time for the mobile terminal.
Another common technique for detecting the output power includes measurement of the current consumed by the power amplifier. This technique is limited to power amplifiers operating in a saturated mode, in which the amplifier DC current is a strong function of the output power. This technique is not applicable to power amplifiers operating in a linear mode, since the amplifier DC current does not change as a function of the output power. For the case of a saturated amplifier, the measured current is directly related to the output power generated by the power amplifier, and is also fed back to the control circuitry. This technique is also disadvantageous due to the loss associated with the current measurement. Current measurement generally requires that a series “dropping” element be added between the associated battery and the power amplifier bias input. The voltage across this element will determine the current entering the power amplifier for a known resistance across the element.
Thus, there remains a need for a new and improved technique for power detection in RF power amplifiers.